The support of well strings within a wellhead generally has utilized gripping elements or slips supported on a bowl which tapers downwardly and inwardly so that as the slips engage the string and assume the weight they are pulled downward on the bowl taper and caused to move into tighter engagement with the string.
A problem which has been encountered with such hangers, particularly when they are supporting heavy strings of high strength casing, is that the weight is sufficient to cause undue inward force on the string which is sufficient to cause deformation of the portion being gripped by the slips beyond the normal engagement of the slip teeth into the exterior of the string.
An example of a prior art device which attempts to solve this problem is the U.S. Pat. No. 2,920,909. This patent discloses a hanging device which includes a slip bowl having a lower inner taper which is at a slight angle to the axis of the unit and an upper taper at a substantial angle to the axis of the unit. Slips are supported in the lower taper and connecting means engage the lower slips, extend through openings in the bowl, through a packer on the upper bowl surface and through the upper slips which are supported by the packer. The unit is designed to avoid severe deformation of the string due to heavy loading. As the lower slips assume the string weight they move downwardly into tighter engagement with the string and also pull the upper slips downwardly causing the packer to be set and thereafter moving the upper slips inwardly against the string to share in the load.
Difficulty is encountered in the use of such units, particularly where higher mud temperatures are encountered, in that the packer is heated and tends to extrude. The failure of the packer is an unacceptable condition to present day drilling operations.
Prior patents have disclosed slip structures in which multiple camming surfaces are provided on the bowl and on the backs of the slips with all of the surfaces being at the same angle to the axis of the unit. Examples of such structures are disclosed in U.S. Pat. Nos. 2,582,700; 3,311,168, 4,390,186 and 4,494,778.
None of these prior art structures has solved the problem of supporting the progressively heavier strings without creating unacceptable deformation of the string.